The price of oil is zooming up due to environmental problems such as global warming, the apprehension of the exhaustion of oil and the supply situations of oil countries, and the development of non-petroleum resins is needed. Especially polylactic acid has the potential for substituting petroleum resins and is characterized by optical properties such as transparency and a low refractive index, and the development of its applications making use of these characteristic properties is expected.
However, polylactic acid generally has a low melting point of about 160° C. and involves a problem with heat resistance such as melting or deformation. Since polylactic acid also has a problem with the stability of physical properties that its biodegradation and deterioration in a wet heat environment proceed at a relatively high speed, its application is limited.
Meanwhile, it is known that stereocomplex polylactic acid is formed by mixing together poly (L-lactic acid) and poly (D-lactic acid) in a solution or molten state (Patent Document 1 and Non-patent Document 1). An interesting phenomenon that this stereocomplex polylactic acid has a melting point of 200 to 230° C. which is much higher than those of poly (L-lactic acid) and poly (D-lactic acid) and shows high crystallinity is found.
There are proposed industrial, agricultural and packaging films which can be used for high-temperature processing and heat-resistance applications and have improved biodegradability and deterioration in a wet heat environment by using this stereocomplex polylactic acid (Patent Document 2).
However, it is difficult to produce a transparent film from the stereocomplex polylactic acid for the following reasons.    (1) The stereocomplex polylactic acid is a composite composition having a poly(L-lactic acid) phase and a poly (D-lactic acid) phase (may be referred to as “homo-phase” hereinafter) and a stereocomplex polylactic acid phase (may be referred to as “complex-phase” hereinafter). In DSC measurement, generally, two peaks which are the melting peak of a crystal having a low melting point of 190° C. or lower corresponding to the melting peak of a homo-phase crystal and the melting peak of a crystal having a high melting point of 190° C. or higher corresponding to the melting peak of a complex-phase crystal are observed. It is not easy to produce a transparent film due to the existence of crystals having different melting points.    (2) The stereocomplex polylactic acid is a crystalline polymer and its crystals are easily oriented. That is, to form the stereocomplex polylactic acid into a film, it must be dried in advance to prevent its hydrolysis. When it is dried by heating in a hot air oven, the stereocomplex polylactic acid readily crystallizes. Also, when the stereocomplex polylactic acid is let pass through the melt preheating zone of an extruder to be melt extruded, the stereocomplex polylactic acid crystallizes and a homo-phase crystal and a complex-phase crystal are grown.
When both the homo-phase crystal having a low melting point and the complex-phase crystal having a high melting point are existent, this mixture is existent in two states which differ in rheological characteristics even when special attention is paid to melting and kneading. Thereby, a sea-island structure or a layer structure of a high-melting point portion and a low-melting point portion due to flow alignment is formed, and flow nonuniformity occurs due to the separation of the two phases, causing optical scattering and making it difficult to obtain a uniform film having a high transmittance.
Like the stretched film described in Patent Document 2, when the draw ratio is high, transparency degrades due to crystalline orientation. A film having a low oriented crystal rate like an unstretched film is readily whitened by heating and has a problem that it is wholly or partially whitened by molding including heating though it is transparent at room temperature. An optical film which is obtained from the stereocomplex polylactic acid, has such high transparency that it can be used for optical purposes and can retain transparency even when it is exposed to a high temperature at the time of processing is desired.    (Patent Document 1) JP-A 63-241024    (Patent Document 2) JP-A 2007-119553    (Non-patent Document 1) Macromolecules, 24, 5651 (1991)    (Non-patent Document 2) Introduction to Chemistry, No. 39, 1998 (published by the Academic Society Publishing Center)